The rapidly growing market of personal communication systems, radio medical implanted systems, and wireless hearing aids provides an increasing demand for more integrated and more efficient radio frequency (RF) integrated circuits (IC's). These IC's are required to operate with supply voltages under 2 V and sometimes down to 1V with minimum current consumption at frequencies up to several GHz.
The recent progress of CMOS technology has considerably improved the transmit freqeuncy of the CMOS devices and made CMOS technology a viable choice for RF integrated circuits allowing cost effective one chip solution.
The transmitter is one of the mostly power hungry blocks of wireless systems. For current saving it is beneficial to use a directly modulated voltage-controlled oscillator (VCO) that operates at the transmit frequency as a signal generator for the transmitter. The modulated signal from the VCO can be amplified by the output stage and applied to an antenna directly or via a filter.
This architecture is simple but very flexible since the output stage can control the output power over a wide range and the antenna parameters can vary with application.
A good example of this transmit architecture is presented by “1 GHz FM transmitter” operating in any 26 MHz band from 100-1000 MHz described in document NT2800 CHIP-MITTER, www.numatechnologies.com/pdf/NT2800.
In some cases when the antenna is made as a high Q inductor with the self-resonant frequency more than 70% above the transmit frequency it is possible that VCO itself can operate as a transmitter. It allows one to save the current used in the output stage.
In this solution the output power range is limited by 12-20 dB depending on the antenna parameters and the supply voltage. This is because the high power is determined by voltage supply while the low power is defined by the minimum level of sustained oscillations.
The major disadvantage of the transmit VCO is that the modulation index tends to depend on the transmit power. This is because the varactor cell based on a MOS capacitor has a relatively narrow voltage control range that closely correlates with the MOS transistor threshold voltage. The typical range is +/−0.5 V while the voltage swing across the tank could be up to several volts. Due to this fact the VCO frequency is sensitive not only to the control voltage applied to the varactor cell but also to the voltage swing across the VCO tank. This fact causes either the power range limitation or the index modulation variations.
The most popular CMOS VCO configurations are based on the differential approach (Andreani, S. Mattisson “On the use of CMOS varactors in RF VCO's,” IEEE J. Solid States circuits, vol. 35, pp. 905-910, June 20) in which two differentially connected varactors or a complete varactor bridge are used for the frequency control (FIG. 1).
The differential approach provides the most efficient way to get high power and S NR under voltage supply limitation. Also control voltage is applied to the nodes with zero RF voltage so control voltage source does not cause additional losses in the LC-resonator.
The varactor block can be connected to the LC-tank circuit directly or via coupling capacitors (U.S. Pat. No. 6,621,365) intended to reduce the VCO gain (FIG. 2).
These coupling capacitors also reduce RF voltage swing across varactors and help to suppress frequency to power sensitivity.
In the direct modulation VCO modulation can be obtained by applying a modulation voltage to the varactor block in addition to the control voltage which sets the carrier centre frequency (FIG. 3).
The disadvantage of this approach is illustrated is that the high VCO gain (˜10 MHz/V) required for the PLL (phase locked loop) means that the modulation voltage swing necessary for peak-to-peak frequency deviation (0.5-2 MHz at 400 MHz) should be 0.05-0.2 V peak-to-peak. Too low level of modulation voltage makes it difficult to control modulation index accurately. Also this low modulation voltage level means low signal to noise ratio. VCO gain variation with RF voltage swing (RF power) also affects modulation index.
Another way to provide modulation is to add an additional varactor block, which is connected to the LC-tank in parallel to the varactor block used for carrier frequency control. The additional block has a separate control voltage input and lower gain (FIG. 4).
The lower gain permits a higher modulation voltage and a larger signal to noise ratio. The modulation index can be controlled not only by modulation voltage swing, but in addition by means of variable variable capacitance sensitivity of the modulation varactor block, leaving appropriate high gain required for PLL loop.
As mentioned above the lower gain can be achieved with the varactor block connected to the LC-tank via coupling capacitors. But coupling capacitors can not be used to trim the gain in the modulation varactor block because both their pins supplied with rather high voltage and CMOS switches used to connect coupling capacitor segments are less effective due to smaller gate to source voltage and higher resistance while thy will be in the RF current path. So they kill the resonator Q-factor or add too much parasitics. Switched capacitors connected parallel to the modulation varactor block are also not suitable for the same reason.
A known way (Chi-Wa Lo, H. C. Luong “A 1.5V 900-MHz Monolithic CMOS Fast-Switching Frequency Synthesizer for Wireless Applications”, IEEE J. Solid-State Circuits, vol. 37, No 4, pp. 459-470, April 2002) is used to trim RF carrier frequency coarsely with switched capacitors connected between the VCO outputs and ground (FIG. 5).
This solution is not suitable for trimming the modulation varactor block sensitivity because these switched capacitors affect the total capacitance connected to the LC-tank, but do not affect the dC/dV. It might seem that the modulator gain would still be affected by these capacitors since frequency depends on relative tank capacitance deviation dC/C which depends also on the denominator, but the PLL actually keeps total LC-tank capacitance constant for certain RF carrier frequency, so the denominator is actually constant.
A variable modulator capacitor with trimmed gain can be obtained with solution shown in FIG. 6, where top pins of two arrays of switched varactors are connected to the LC-tank via coupling capacitors. Modulation voltage is applied to these top pins relative to ground via resistors. Bottom pins of the switched varactors can be connected to ground via NMOS switches depending on required modulator gain. The disadvantage of this solution is incomplete utilization of the varactor's C-V characteristic since only positive modulation voltage can be applied relative to ground. Also full power RF voltage and control voltage are applied to the same top pin of the varactor and resistors used to decouple modulator voltage source from RF voltage cause additional unwanted losses and parasitics in the LC-tank.